Various grades of stainless steel are commonly used in the construction of equipment used in manufacturing applications. For example, in the pharmaceutical industry, stainless steel is used in equipment used for the manufacture of pharmaceuticals. This equipment includes, but is not limited to, process vessels, piping, water systems and ancillary equipment. The most common grade of stainless steel used in pharmaceutical manufacturing equipment is Type 316L stainless steel. In such applications, it is particularly important to avoid the introduction of contaminants into the manufacturing process.
Typically, a “passive layer” (e.g., an oxide) is applied to the surface of the stainless steel by a passivation process. “Passivation” includes the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of a protective passive film. The passive layer is a thin, adherent film or layer formed on the surface of the stainless steel as a protective coating to protect the underlying surface from further chemical reaction, such as corrosion, electrodissolution, or dissolution.
During the normal life of the stainless steel equipment, the passive layer of the stainless steel is worn away, thereby allowing corrosion to occur on the surface of the stainless steel due to the presence of iron oxide. This type of corrosion is commonly referred to as “rouge,” since it may appear as a reddish layer on the surface of the stainless steel. Rouge corrosion may be localized corrosion limited to only a portion of the stainless steel surface, or may be a general corrosion encompassing the whole stainless steel surface.
Rouge corrosion must be removed periodically from the surface of the stainless steel in order to prevent contamination of chemicals (e.g., pharmaceutical products or intermediate products) prepared in the stainless steel equipment. The corrosion also needs to be removed prior to passivation or repassivation of the stainless steel equipment.
“Derouging” is a process wherein contaminants (e.g., exogenous iron or iron compounds) are removed from the surface of stainless steel by means of a chemical dissolution. It should be appreciated that derouging is an ongoing maintenance issue for manufacturers (e.g., pharmaceutical companies) that operate stainless steel systems with high purity water running at elevated temperatures. This includes, but is not limited to, vapor compression stills, multi-effect stills, clean steam generators, steam piping systems, WFI (water for injection) and purified water systems operating above ambient temperatures.
A typical “derouging” process includes treating the equipment with an acid solution that removes the surface contamination, but will not significantly affect the stainless steel itself. The acid solution may include, but is not limited to, mineral acids (e.g., nitric acid or phosphoric acid), organic acids (e.g., citric acid), etc. These acids can be formulated products or neat solutions. The acid solution is recirculated or otherwise brought into contact with the “rouged” surface of the stainless steel.
The progress of a derouging process is typically monitored by taking samples of the acid solution contacted with the surface of the stainless steel, and analyzing the samples for iron content. When the iron level in the acid solution reaches a maximum, it is presumed that the majority of the rouge corrosion has been removed from the surface of the stainless steel. As additional acid is added to the acid solution, the increase in the iron level in the acid solution will slow as the amount of remaining rouge corrosion is reduced. The derouging process is considered complete when the amount of iron entering the acid solution has declined to a relatively small amount. Throughout a typical derouging process, samples of the acid solution are repeatedly taken to a laboratory and analyzed for iron content.
A repassivation process follows the derouging process. The repassivation process includes treating the stainless steel with a passivating solution to provide a chromium rich surface layer. The passivating solution may include, but is not limited to, mineral acids (e.g., nitric acid or phosphoric acid), organic acids (e.g., citric acid), etc. These acids can be formulated products or neat solutions.
The passivating solution is recirculated or otherwise brought into contact with the derouged surface of the stainless steel. The passivating solution selectively dissolves iron from the top layer of the stainless steel to make the iron less available at the surface to form iron oxides. The passivating process removes the iron, and increases the concentration of chromium at the surface of the stainless steel. Iron levels can be monitored throughout this process in the same manner as described above in connection with the derouging process. The iron levels in the solution can be correlated with the progress of the passivation process.
Prior art processes for monitoring iron and chromium levels for the above-described processes are time consuming, do not occur in real time, and do not facilitate automated processes.
The present invention addresses these and other drawbacks to provide a method and apparatus for monitoring the concentration of metallic cation concentrations in a solution.